1. Field of the Invention
The present invention generally relates to an antenna apparatus, and more particularly, to an antenna apparatus having a plurality of feeding-radiating elements.
2. Description of the Related Art
In recent years, the number of cellular telephones using a plurality of frequency bands has increased. Such cellular telephones switch from one frequency band in which telephone traffic concentration occurs to another frequency band to achieve smooth telephone communication. The cellular telephones of this type require an antenna which is excited in two frequency bands. For example, U.S. Pat. No. 6,333,716 discloses an antenna for use in GSM (Global System for Mobile Communications) cellular telephones which is excited at frequencies in the 900 MHz and 1800 MHz bands.
This type of antenna includes a metallic pattern disposed on a dielectric housing, and a slit formed in the metallic pattern to form two feeding-radiating elements having different electrical lengths, wherein a signal current fed from a common feeding point causes one of the feeding-radiating elements to be excited at a frequency in the 900 MHz band and causes the other feeding-radiating element to be excited at a frequency in the 1800 MHz band.
However, typically, when a current is fed from a common feeding point to a plurality of feeding-radiating elements, in a frequency band allocated to each of the feeding-radiating elements, sufficient radiating resistance may not be maintained for each feeding-radiating element because each of the feeding-radiating elements does not experience the optimum electrical length from the feeding point to the feeding-radiating element, thereby making the bandwidth for resonance narrower. Another problem arises in that insufficient signal power supply resulting from no impedance matching between each of the feeding-radiating elements and the signal source causes insufficient gain of the feeding-radiating elements, or causes variations in gain from one feeding-radiating element to another.
In order to overcome the problems described above, preferred embodiments of the present invention provide an antenna apparatus having a plurality of feeding-radiating elements, wherein excellent electrical matching is achieved for each of the feeding-radiating elements.
According to a preferred embodiment of the present invention, an antenna apparatus includes a dielectric base, a plurality of feeding-radiating elements each including a feeding electrode and a radiating electrode which are disposed on surfaces of the base, and a substrate which fixedly supports the base, wherein a common feeding point for feeding a current to the plurality of feeding-radiating elements is disposed on the substrate, a stub continuously expanding from the feeding point is disposed on a surface of the substrate, or on a surface of the base and a surface of the substrate, and the feeding electrodes of the plurality of feeding-radiating elements are connected to matching points of the stub which are determined based on the effective line length of the radiating electrodes.
The feeding-radiating elements are excited at the resonance frequency which depends upon the effective line length of the radiating electrodes. Since the feeding electrode of each of the feeding-radiating elements is connected to the matching point of the stub which has the optimum stub length for each feeding-radiating element, each feeding-radiating element can achieve an excellent resonance property at the resonance frequency, while the required bandwidth can be maintained in the frequency band to which the resonance frequency belongs.
The stub length optimization for each of the feeding-radiating elements allows the optimum impedance matching between the feeding-radiating elements and the feeding point or the signal source, thereby allowing the maximum power to be supplied from the signal source to the feeding-radiating elements to increase the gain of the feeding-radiating elements. The effective line length L of a radiating electrode is expressed by L=xcex/4{square root over (∈)}, where ∈ denotes the effective relative dielectric constant of a base, and xcex denotes the wavelength of resonance frequency. As used herein, xe2x80x9ca surface of a basexe2x80x9d indicates at least one surface of a three-dimensionally shaped base. The stub may be a short stub or an open stub, and is disposed on a surface of a substrate, or on a surface of the substrate and a surface of a base.
Preferably, a radiating electrode without a feeding electrode is arranged on a surface of the base so as to be adjacent to the radiating electrode of at least one of the plurality of feeding-radiating elements.
The radiating electrode without a feeding electrode defines a parasitic radiating element. The parasitic radiating element is electromagnetically coupled with a feeding-radiating element adjacent thereto, and is thus energized to resonate at a frequency in the same frequency band as that of the resonance frequency of the adjacent feeding-radiating element. Accordingly, dual resonance matching can be achieved between the resonance frequency of the feeding-radiating element and the resonance frequency of the parasitic radiating element, and the frequency bandwidth for the dual resonance can thus be broader than the frequency bandwidth for the resonance by the feeding-radiating element alone.
The stub may be a short stub of which a portion far from the feeding point is coupled to the ground.
Therefore, the optimum reactance which is expressed by the stub length using the ground potential as a reference for each of the feeding-radiating elements can be applied to the feeding-radiating elements. Then, the optimum resonance matching can be achieved for each of the feeding-radiating elements. For example, a longer stub length may be set for a feeding-radiating element having a lower resonance frequency, while a shorter sub length may be set for a feeding-radiating element having a higher resonance frequency, thereby achieving the optimum impedance matching between each of the feeding-radiating elements and the feeding point.
The antenna apparatus further includes a ground conductive layer provided on the substrate. The stub may be an open stub which is separated from the ground conductive layer by a slit formed in the ground conductive layer.
The reactance to be applied to each of the feeding-radiating elements is provided based on a distance from a feeding point of the open stub to the feeding electrode of each of the feeding-radiating elements. Therefore, the feeding-radiating elements can have an electrical length which achieves resonance property optimization at a prescribed frequency band.
A reactor may be connected between the stub and the ground conductive layer.
Since the stub is partially formed of a lumped element such as a reactor, for example, an inductor or a capacitor, the effective stub length can be changed by selecting the reactance of the lumped element. When reactance is applied to an open stub, the stub will be a short stub.
The reactor may include a pattern electrode having a reactance component which is disposed on a surface of the base.
As a result, the stub length can be changed without using a lumped element. The reactance of the pattern electrode can be changed by changing the length, width, or configuration of the pattern electrode. The pattern electrode can be provided on a surface of the base together with a feeding electrode, and the pattern formation can thus be readily performed.
The stub may include a feeding land having a feeding point provided on the substrate, and a stub pattern which is disposed on a surface of the base and which is connected to the feeding land.
The feeding electrode of each of the feeding-radiating elements may be integrally connected beforehand at the position of a matching point of the stub pattern on the base. When one end of the stub pattern is connected to the feeding land, final matching between each of the feeding-radiating elements and the feeding point or supply source is achieved. The stub pattern may be a short stub by coupling to the ground the end of the stub pattern opposite to the end which is connected to the feeding land. Alternatively, the stub pattern may be an open stub by making the opposing end open. The optimum stub length from the matching point to the feeding electrode of each of the feeding-radiating elements can be changed by changing the length and width of the stub pattern.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.